Hydrogen absorbing properties of ScM2 Laves phase alloys (M = Fe, Co and Ni)

Journal of

~YS AHD COM~OUHD5 ELSEVIER

Journal of Alloys and Compounds 226 (1995) 75-80

Hydrogen absorbing properties of ScM2 Laves phase alloys (M = Fe, Co and Ni) M. Yoshida a, E. Akiba b a Hitachi Chemical Co., Ltd. Tsukuba Research Laboratory, 48 Wadai, Tsukuba, lbarala" 300-42, Japan National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, lbarala" 305, Japan Received 10 January 1995

Abstract

Hydrogen absorbing properties of ScM2 (M = Fe, Co and Ni) Laves phase alloys were studied by pressure-composition (PC) isotherm measurements at the temperature range from 293 K to 353 K. Crystal structural change of these alloys during hydrogenation was also studied by in situ X-ray powder diffraction measurements under hydrogen pressure up to 4 MPa. Clear plateaus were observed in all the absorption and desorption isotherms. The enthalpy of formation of ScM2Hx (M = Fe, Co and Ni) was estimated from the equilibrium plateau pressure observed in the PC isotherm to be -28(1), -30(1) and -16(2), respectively, in units of kJ per mol H2. In situ X-ray diffraction measurements revealed that the hydrogenation did not produce any change in metal substructure or any indication of amorphization in the ScM2-H2 systems (M =Fe, Co and Ni). The cell volume, however, increased by 23.1% (Fe), 21.0% (Co) and 14.5% (Ni) upon hydrogenation. Keywords: Hydrogen absorption; Laves phase alloys; Pressure-composition isotherms

1. Introduction

Recently, some AB2 Laves phase alloys were intensively investigated in the field of metal hydrides. The Laves phase alloys are categorized into three structure types; MgZn2-type structure (C14), MgCu2-type structure (C15) and MgNi2type structure (C36) [ 1]. ZrMn2-based alloys with C14 attracted attention as second generation materials for not only nickel-hydrogen secondary batteries but also other applications owing to their larger hydrogen absorbing capacity than ABs-type hydrogen absorbing alloys such as LaNi5 alloys [2]. Although the advantage of the Zr-based Laves phase alloys was large hydrogen capacity, their hydrides, for example ZrMnzH3 [ 3 ] and ZrV2H4.5 [4 ] appeared to be very stable [ 5,6 ] and did not reversibly react with hydrogen around room temperature. The hydride system operating under ambient hydrogen pressure is the first requisite for applications such as the nickel-hydrogen battery. Therefore, many efforts were made to control a hydrogen plateau pressure by substitution of constituent elements in these Zr-based alloys [7]. For example, the hydrogen plateau pressure rose on the substitution of Ni in the Mn site of ZrMn2 [8]. However, Y and rare earth metal-based Laves phase alloys were also known to absorb large amounts of hydrogen; however, their hydrides were also shown to be very stable [9]. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI0925-8388(95)01611-2

Among the binary Laves phases, only ScFe2 and TiMnL5 are reported to show reversible reactions around room temperature without substitution of other elements, and to have high hydrogen capacities close to 1 in terms of the hydrogen to metal ratio, H/M [10,11]. Other Sc-based alloys, for example ScCo2 and ScNi2 are expected to show behavior similar to that found in the ScFe2-H2 system. However, their hydrogen absorbing properties (pressure-composition isotherm and enthalpy of hydride formation) have not been reported in detail. Furthermore, among the hydrogen absorbing alloys, only ScFez [ 12] has C36 and the others have C15 or C14 to our knowledge. The peculiarity of the structure is that there are six different tetrahedral sites for hydrogen to occupy as revealed by the neutron diffraction study of ScFe2H3 [ 13 ]. The structural characterization of hydrides of ScCo2 and ScNi2 has not been reported in detail; however, if their hydrogen site preferences are similar to that of ZrV2H4.5 [4] with C15, the structure of ScCo2 and ScNi2 will contain two attractive tetrahedral sites for hydrogen. Therefore, it would be very interesting to compare its hydrogen absorbing property and crystal structural change upon hydrogenation with that of ScCo2 (C15 [ 14] ) and ScNi2 (C15 [ 15] ), and thus investigate the effect of the structural difference on these properties.

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M. Yoshida, E. Akiba / Journal of Alloys and Compounds 226 (1995) 75-80

One of the purposes of this paper is to study the hydrogen absorbing properties of ScM2 Laves phase alloys (M = Fe, Co and Ni) by pressure-composition (PC) isotherm measurements, and to estimate the enthalpies of formation and decomposition of their hydrides. Another purpose is to investigate crystal structural change upon hydrogenation by in situ X-ray diffraction measurements.

10

---

1

a,

0.1

2. Experimental ScM2 (M=Fe, Co and Ni) alloys were prepared by arc melting from 99.9% pure metals in argon atmosphere. The homogeneous samples were obtained by turning out and remelting the alloys four times. It is very important for synthesis that the alloy on the water-cooling plate in the arc melted furnace is turned out as quickly as possible, and re-melted before it cools, otherwise it will be broken, because an intermetallic compound that is usually brittle is quite easily broken during cooling. X-ray powder diffraction data of all the alloys were measured on a RIGAKU RAD-A diffractometer with Cu Ka radiation. The pressure-composition (PC) isotherms were measured by a conventional constant volume apparatus [ 16]. Before PC isotherm measurements all the alloys were activated by evacuating at 623 K for 3 h and by introducing hydrogen up to 5 MPa at room temperature. This treatment was repeated three times. In situ X-ray diffractionprofiles of all the samples (internal standard: silicon powder with a = 5.4308 ,~) were measured on a RIGAKU Rotaflex RU-200 X-ray diffractometer (Cu Ka radiation) equipped with a specially designed autoclave in which hydrogen can be pressurized up to 5 MPa and the temperature can be controlled from room temperature to 727 K [17].

3. Results 3.1. Pressure--composition

isotherm

measurements

All the alloys were measured by X-ray powder diffraction for phase analysis. ScFe2 showed the X-ray diffraction patterns of the hexagonal C36 ( P 6 3 / m m c ) [12]. ScCo2 and ScNi2 were confirmed to contain the cubic C15 Laves phase [ 14,15]. No impurity phase was observed in any of the diffraction patterns. In order to investigate the hydrogen absorbing properties of the ScM2 alloys (M=Fe, Co and Ni), the PC isotherm measurements were performed in a temperature range from 293 K to 353 K (ScCo2: 293, 303 and 313 K, ScFe2: 313, 333 and 353 K, ScNi2: 303, 313 and 323 K ). Fig. 1 shows the PC isotherms of the ScM2-H2 systems (M = Fe, Co and Ni) at 313 K. Clear plateaus were observed in all the absorption and desorption isotherms. ScCo2 and ScFe2 absorbed a

0.01

, ,

0

I,,,

0.2

I ~ ~ , I,

0.4

0.6

I , , ~

0.8

t

1

,

,

~

1.2

Hydrogen content (H/M) Fig. 1. The pressure-composition isotherms of ScM2 Laves phase alloy (M=Fe (O, absorb; @, desorb), Co (E], absorb; II, desorb) and Ni (A, absorb; A, desorb)) at 313 K.

large amount of hydrogen up to compositions ScCo2H3.o3 (the ScC02H3 phase, 1.88 wt.%) and ScFe2H295 (the ScFe2H3 phase, 1.82 wt.%) at 4 MPa. The hydrogen content of ScNi2H~.95 (ScNi2H2 phase, 1.22 wt.%) was less than that of the other hydrides. All the alloys were found to reversibly react with hydrogen around room temperature as shown in Fig. 1. ScFe2H3 has almost fully decomposed to ScFe2Ho.o3 (at 0.01 MPa and 313 K), while the hydrogen content of the ScC02-H2 and ScNi2-H2 systems under the same conditions were 0.40 and 0.35 hydrogen atom per formula, respectively. The wide solubility range of the hydride phase and the narrow plateau region were shown in the isotherms of the ScFe2-H2 system compared with the ScM2-H2 (Co and Ni) systems. Large hysteresis was observed in the isotherms of the ScM2H2 ( M = C o and Ni) system. Conversely, the hysteresis observed in the ScFe2-H2 was extremely small. In the preliminary experiments, it was found that the large hysteresis reproducibly appeared in the PC isotherms of the ScM2-H2 ( M = C o and Ni) system, and the same pressure hysteresis was observed. In this work, the samples after the PC isotherm measurement were evacuated again at 623 K for 3 h in order to be used for the measurement at a different temperature. This treatment was the same procedure as the activation of the alloys. Therefore, they were considered to be always in an initial state before each PC measurement. The enthalpies of formation and decomposition on the ScFe2-H2, ScC02-H2 and ScNi2-H2 systems were calculated from the results of PC isotherm measurements. The equilibrium plateau pressures of both absorption and desorption are defined as the equilibrium pressure at the center of the plateau region. The plateau pressure can be expressed by Eq. (1) B In PH2= A + T

( 1)

where the constants A ( - A S ~ R , R is the gas constant) and (AH/R) a r e associated with the entropy and enthalpy change of one mole of hydrogen. A plot of the equilibrium

B

M. Yoshida, E. Akiba / Journal of Alloys and Compounds 226 (1995) 75--80

10

....

i , ' ' ' i ....

that of ScNi2H 2 ( - 16(2) kJ per mol H2) was about half of others. The result of the ScFez-H2 system agrees with the literature data ( - 30(3) kJ per tool H2 [ 10] ).

i , ' , , [ ' ' , ' i , , ' ' r , , , ' A ~

77

A~

3.2. In situ X-ray diffraction measurement

0.1

0.01 2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

1/T x 10-3 ( l / K ) Fig. 2. Plot of log PH2 against 1 / T f o r ScM2 Laves phase alloy ( M = Fe (©, absorb; O, desorb), Co ([], absorb; II, desorb) and Ni ( A , absorb; A, desorb) ). Table 1 The enthalpy of formation and decomposition of ScM2 hydrides ( M = F e , Co and Ni) Hydride system

Formation

Decomposition

ScFea-H2 ScCo2-H 2 ScNi2-H2

- 28 ( 1 ) kJ per mol H2 - 3 0 (1) kJ per mol H2 - 16 ( 2 ) k J per mol H E

- 30 ( 1 ) kJ per mol H2 - 3 1 (1) kJ per mol H2 - 16 (2) kJ per mol H2

under reduced pressure I Si

I 311 II

PH:! = 4.0 MPa

•

r

Si

21

"1 Si |

311

30

40

~

Si

49

2O Fig. 3. In ~itu X-ray diffraction patterns of the ScNi2-H 2 system at room temperature ( O , peaks originated from sample holder).

plateau pressure (lnPH2) vs. inverse of temperature ( 1/T), a van't Hoff plot, is shown in Fig. 2. The enthalpy of hydride formation, AH, was calculated by the least squares method with the slope of each line in Fig. 2. Table 1 lists the enthalpy of formation and decomposition of hydrides. The enthalpy of formation of ScFe2H3 ( - 2 8 ( 1 ) kJ per tool H2) was very close to that of ScCo2H3 ( - 30(1) kJ per mol H2); however,

In order to investigate changes in crystal structure during hydrogenation-dehydrogenation, in situ X-ray diffraction measurements were performed at room temperature. Fig. 3 shows the diffraction patterns of ScNi2, its hydride (at 4 MPa) and its dehydride (at 0.01 MPa). During the hydrogenation, diffraction peaks shifted towards the lower angles and were broadened. No extra peak was observed in the diffraction patterns of the hydride of ScNi> The hydrogenation can be said to have kept the metal substructure unchanged, but led to lattice-expansions (ScNi2-H2:6.923 (3) ,a, for alloy and 7.242(6) k, for hydride, ScFe2-H2: a = 4.974(4) .~ and c=16.28(1) ,~, for alloy, a=5.34(1) A and c = 17.40(4) ,~ for hydride; ScCo2-H2: a=6.927(2) ,a, for alloy, a = 7.381 (6) ~ for hydride). The cell volume of ScNi2 increased by 14.5% after hydrogenation. The AV/VofScNizH2 was about two-thirds of the others (ScFez-H2: 23.1%, ScCo2-H2: 21.0%). This ratio corresponded to that of the hydrogen content of ScNi2H2 and ScFezH3 (or ScCo2H3). Consequently, the volume increase per hydrogen atom is almost identical among the three hydride systems (ScFe2H3; 3.4 j~3per H atom, ScCo2H3; 2.9 ~3 per H atom and ScNi2H2; 3.0 ~3 per H atom). No sign of amorphization was observed in this experimental condition for the ScM2-H2 systems (M = Fe, Co and Ni), in contrast to the case of Y and rare earth metal-based Laves phase alloys that formed hydrides of crystalline or amorphous depending on hydrogen pressure [ 18]. On dehydrogenation (at 0.01 MPa), all peaks went back to the same position as they were before; however, the peaks were still broadened as shown in Fig. 3. Then, the dehydrided ScNi2 was heated up to 623 K at 0.01 MPa (with pumping). This heat treatment was the same procedure as the activation of the alloys before the PC isotherm measurement. After the heat treatment, the peaks became as sharp as that of the starting alloys. In order to quantitatively estimate the peak broadening effect, the X-ray diffraction profiles of the starting alloy, the dehydride at 0.01 MPa and the dehydride after heat treatment were analyzed by the Rietveld method with refinement program RIETAN-92 [ 19]. Full width at half maximum (FWHM), which is one of the structural parameters obtained from the Rietveld refinement, is a good indicator of the diffraction peak broadening. The FWHMs in terms of 2 0/degree for the strongest 3 1 1 peak were 0.153 for starting alloy, 0.340 for dehydride after reduced pressure and 0.177 for heattreated sample. This peak broadening was also observed in the diffraction pattern of the hydride of ScCo2. However, the peak broadening was not observed in the diffraction pattern of ScFe2 hydride with C36 as shown in Fig. 4.

78

M. Yoshida, E. Akiba / Journal of Alloys and Compounds226 (1995) 75--80

r

PH2= 4.0 MPa

114 II

I S'

t202

Si

Alloy

114

•

28

si

•

t

I

30

40

I

/

~202 II

Si

50

2O Fig. 4. In situ X-ray diffraction patterns of the ScFe2-H2system at room temperature (@, peaks originated from sample holder).

4. Discussion

The PC isotherm measurements indicated that the hydrogen absorbing property of ScFe2 with C36 remarkably differs in terms of the hysteresis and the width of solubility range from other alloys with C15. These differences are considered to be associated with the crystal structure of the host alloys. Hydrogen goes into two different tetrahedral A2B2 and AB3 sites in the AB2 alloy with C15 such as ZrV2 [4], while the ScFe2 host alloy with C36 has six different tetrahedral A2B2 sites (two Sc and two Fe) [ 13]. Didisheim et. al [20] have found that only the hydrogen A2B2 site occupancy increases as a function of hydrogen concentration up to about 1 in terms of H/M ratio in the structure of ZrV2H4.5, and there exists the preference of the tetrahedral A2B2 hydrogen site (two Zr and two V) over the tetrahedral ABa site (Zr and three V). If in the ScCo2H3 and ScNi2H2 hydrogen occupies the A2B2 sites in the same manner as ZrV2 hydride, the enthalpy of hydride formation estimated above, AH, presumably corresponds to the enthalpy, Mar', for hydrogen to enter the A2B2 sites. Considering wide plateau regions observed in the PC isotherms of ScCo2-H 2 and ScNi2-H 2, there should be a hydrogen site preference in their structures as found in that of ZrVzH4.5. In the case of ScFe2H3, if the AH' for all the A2B2 sites is the same, the wide plateau region is presumably observed; however, the result indicated the wide solubility range of the hydride phase and narrow plateau region. Accordingly, the AH' values for the six A2B 2 sites are assumed to be slightly different from each other. However, for further discussions, neutron diffraction measurements for different hydrogen contents of ScFe2 hydrides will be required. We will compare the hydrides of ScM2 (M=Fe, Co and Ni) with other Laves phase hydrides such as ZrM2Hx and YM2Hx from the viewpoint of the stability of hydride.

Hydrides of ZrFe2 and ZrCo2 showed quite small hydrogen capacities of less than 0.2 hydrogen atom per formula as reported by Pebler et al. [6]. In this case, the plateau seemed to exist at the higher pressure compared with the experimental limits and only the region of the solid solution was presumably observed in the PC isotherm. The ZrNi 2 intermetallic phase does not exist in the Zr-Ni binary system [21]. YM2 (M=Fe, Co and Ni) alloys absorbed a large amount of hydrogen up to more than 1 in the H/M ratio, but no plateau was observed in their isotherms [9]. Shaltiel et al. [22] postulated that a stable hydride showed a low plateau pressure and a high hydrogen capacity in the PC isotherm. Therefore, it may be concluded that the ScM2 hydrides are more stable than the ZrM2 hydrides, and less stable than the YM: hydrides. We will discuss the results of the enthalpies of hydride formation from the "the rule of reversed stability" [23], which states that the more stable the binary "compound, the less stable the ternary metal hydride. Miedema [ 24 ] reported that in the case where there is no direct A-A (A: hydrogen attracting minority metal) nearest neighbors in the ABn compound; for example ABs-type and AB2-type alloys, the enthalpy of formation is determined by the interaction of A and B. Miedema [24] postulated that the enthalpy of formation of binary alloy could be described by the following equation;

Att=f(c) [ - e e ( A ~b)2+ a(Anw) 2]

(2)

wheref(c) is a symmetrical function of concentration, th is the electronegativity parameter, nw is the density of electrons at the boundary of the Wigner-Seitz cell [23]. P and Q parameters were derived from Ref. [24]. As calculated using Eq. (2), the enthalpy of formation of ScFe2, ScCoz and ScNi2 alloys were - 30 kJ mol- 1, _ 93 kJ mol- 1 and - 175 kJ mol- 1, respectively. From this calculation, ScNi2 was found to be much more stable than others. The result that the ScNizHa phase is less stable follows "the rule of reversed stability". The IAH[ of ScCoz was calculated to be about 3times as large as that of ScFe2. However, the result obtained from the van't Hoff plot indicated that the stabilities of the two hydrides were quite similar. This is supposed to be derived from another factor, for example the difference in the crystal structure between C36 and C15. When we compared two alloys, ScCo2 and ScNi2, both of which have the same C 15, the It till of ScCo2 was roughly twice as large as that of ScNi2. As expected from "the rule of reversed stability", the l anl of ScNizH2 was almost twice as large as that of ScC02H3. Table 2 lists the calculated enthalpy of formation of typical Zr-based, Sc-based and Y-based alloys and the estimated enthalpy of formation of their hydrides. The I AHI of all the alloys increased with increasing atomic number when the same element was adopted as the A site of the AB2 alloy. The IAHI of the Zr-based hydrides decreased in the order Zl~r2H 4 [25] >ZrMn2H 3 [26] >ZrFe2Ho. 2 and ZlCo2no. 2. The [ AH[ of the Sc-based hydrides decreased in the order ScMn2H4 [25] > ScCo2H3 >__ScFezH 3 > ScNi2H2.

M. Yoshida, E. Akiba / Journal of Alloys and Compounds 226 (1995) 75-80

79

Table 2 Comparisonof Sc-basedalloys with Zr and Y-basedalloys Alloy

Cr

Mn

Fe

Co

Ni

Zr ( 1.60 .~)-based hydride Structure (hydride and alloy) AH (kJ per mol): alloy AH (kJ per mol H2): hydride

ZrCr21-I4

ZrMn2H3

ZrFe2Ho.2

C14 - 74 -44.4 [26]

C15 - 84 - [6]

Za'Co2Ho.2 C15 - 142 - [6]

No compound

C15 - 42 - 58.4 [25]

Sc ( 1.64 ~)-based hydride Structure (hydride and alloy) AH (kJ per mol): alloy AH (kJ per mol H2) : hydride

-

ScMn2H4

ScFe2H3

ScC02H3

ScNi2H3

C14 - 30 - 63.0 [25]

C36 - 30 - 28 a

C15 - 93 - 30 a

C15 - 175 - 16 a

Y ( 1.81 ,~)-basedhydride Structure (hydride and alloy) AH (kJ per mol): alloy AH (kJ per mol H2): hydride

-

-

[21 ]

YFe2H4

YCo2H 4

YFe2n,,

C15 -3 n.p. [9]

C15 -70 n.p. [9]

Cl5 - 154 n.p. [9]

a This work. AH values of alloys were calculatedusing Eq. (2) [24]. n.p. meansthat no plateau was observedin the isotherm.

The stability of the hydride having the same element for the A site decreased as a function of the atomic number except for the order for ScC02H3 and ScFe2H 3. In the case where the constituent element of the B site in AB2 alloy is the same, ScM 2 is larger than YM 2, and smaller than ZrM 2 in the calculated I AH[. This with "the rule of reversed stability" would allow us to estimate that the hydride stability increases in the order ZrM2Hx < ScM2H, < YM2H~. The stability of the Laves phase is determined by atomic size as well as electron concentration [ 1 ]. Since the atomic radii of Ni, Co and Fe are the same [27], the hydride stability of AB2 with A fixed is considered to depend on the number of d electrons of the B site elements. However, the number of d electrons of Sc is the same as that of Y, and the atomic radius increases in the order Z~ < Sc < Y [ 27 ]. The atomic size is presumably dominant in determining the hydride stability of AB 2 alloys when the B site is fixed. In situ X-ray diffraction measurements confirmed the presence of the broadening effect in the ScCo2 and ScNi 2 hydride systems with C 15. Nomura et al. reported that a large hysteresis observed in the LaNis-H2 system obviously correlates to the peak broadening [ 16]. In general, broadening of diffraction peaks is known to arise from crystallite-size and microstrain effects [28]. Nomura et al. [ 16] applied the WarrenAverbach method for analysis of the diffraction profile, and concluded that the microstrain was a predominant factor to affect the hysteresis. In the present work, we observed peak broadening when a large hysteresis appeared in the isotherms. This is the same behavior as observed in the LaNis-H2 system [ 16]. The broadening effect and hysteresis are supposed to be correlated to the crystal structure of the host alloy. However, a further investigation is required in order to confirm such correlations between the hysteresis, the microstrain and the crystal structure.

5. Conclusion We studied the hydrogen absorbing properties of S c M 2 ( M = F e , Co and Ni) Laves phase alloys and estimated the enthalpies of the hydride formation. We also investigated crystal structural change with hydrogenation, and discussed the phase stability of the related Laves phase alloys and their hydrides to draw the following conclusions. ( 1 ) The PC isotherm measurements indicate that the ScM2 (Fe, Co and Ni) Laves phase alloys reversibly reacted with hydrogen at around room temperature, and that clear plateaus were shown in all the isotherms. The enthalpy of hydride formation for ScM2 (Fe, Co and Ni) alloys were - 28(1), - 30(1) and - 16(2) KJ per mol H2, respectively. (2) Among the Zr-based, Sc-based and rare earth metalbased Laves phase alloys, only ScFe2 has C36 phase. A smaller hysteresis and a wider region of the hydride phase were observed in the ScFe2-H2 system, as compared with the other Sc-based alloys. (3) We compared the Sc-based AB2 hydrides with typical Zr-based and Y-based hydrides from the viewpoint of the hydride stability. The stability of the AB2 hydride with A fixed is dependent on the number of d electrons of the B element, and when the B element is fixed it is predominantly determined by the atomic size of the A element. (4) In situ X-ray diffraction measurement has revealed that the hydrogenation did not produce any change in metal substructure or any indication of amorphization in the ScM2-H2 systems ( M = F e , Co and Ni). The cell volume, however, increased by 23.1% (Fe), 21.0% (Co) and 14.5% (Ni) upon hydrogenation. The peak broadening effect was found in the diffraction of hydrides of

M. Yoshida, E. Akiba / Journal of Alloys and Compounds 226 (1995) 75-80

80

ScC02 and ScNi2. Conversely, ScFe2 did not show any significant change in the diffraction peak width upon hydrogenation.

Acknowledgments We thank Mr. Y. Ishido of Shin-Kobe Electric Machinery Co., Ltd. and Mr. K. Nomura of the National Institute of Materials and Chemical Research for technical assistance.

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